Recently, a distance measuring method utilizing ultrasonic waves has been used as an accurate distance measuring method. In the method, ultrasonic waves are emitted from an ultrasonic generation device and are caused to impinge on an object to be measured. The ultrasonic waves reflected by the object are detected by an ultrasonic microphone device, and the distance to the object is calculated from the time elapsed between the emission and the detection.
For example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2004-297219) discloses an ultrasonic generation device in which piezoelectric vibrators are attached to a housing. The device in Patent Document 1 is configured as an ultrasonic sensor device in which a single device serves as both an ultrasonic generation device and an ultrasonic microphone device.
FIG. 10 illustrates an ultrasonic generation device (ultrasonic sensor device) 200 disclosed in Patent Document 1. FIG. 10 is a cross-sectional view of the ultrasonic generation device 200. The ultrasonic generation device 200 has a structure in which a first piezoelectric vibrator 102 and a second piezoelectric vibrator 103, which vibrates in opposite phase from that of the first piezoelectric vibrator 102 to cancel unnecessary vibration, are attached to a housing 101. A lead 104 is connected to each of the housing 101, the first piezoelectric vibrator 102, and the second piezoelectric vibrator 103. An inner space of the housing 101 is filled with a flexible filler 105.
To make a measurement result more accurate and to lengthen a measurable distance in the distance measuring method using such an ultrasonic generation device, it is useful to increase an output sound pressure of the ultrasonic generation device.
However, increasing the output sound pressure in the ultrasonic generation device 200 is limited. That is, although increasing the output sound pressure requires that polarization of the piezoelectric vibrator be increased or electric power applied to the piezoelectric vibrator be enlarged, the polarization of the piezoelectric vibrator is limited. Further, if the applied electric power is excessively enlarged, the piezoelectric vibrator exceeds its breakdown limit. Hence, increasing the output sound pressure is limited.
In recent years, there is a strong demand to reduce the sizes of electronic apparatuses and devices. However, if the piezoelectric vibrator is miniaturized to reduce the size of the ultrasonic generation device, a problem arises in that the output sound pressure falls. Therefore, there also is a problem in that size reduction of the ultrasonic generation device is difficult.
Accordingly, the present applicant has addressed the development of an ultrasonic generation device having a high outpour sound pressure, and has succeeded in developing an ultrasonic generation device that has a high output sound pressure with a specific structure. Although a patent application on this ultrasonic generation device has been filed (for example, PCT/JP2011/68095), the application has not been laid open yet at the time of filing the present application.
FIG. 11 schematically illustrates an ultrasonic generation device 300 filed as a patent application (not laid open) by the present applicant. FIG. 11 is a cross-sectional view of the ultrasonic generation device 300. FIG. 11 simplifies and schematically illustrates details.
The ultrasonic generation device 300 includes an ultrasonic generation element 201.
The ultrasonic generation element 201 includes a frame 202, a first piezoelectric vibrator 203, and a second piezoelectric vibrator 204. The frame 202 has a through hole in its center portion. The first piezoelectric vibrator 203 is bonded to a lower principal surface of the frame 202, and the second piezoelectric vibrator 204 is bonded to an upper principal surface of the frame 202.
The first piezoelectric vibrator 203 and the second piezoelectric vibrator 204 are vibrated in mutually opposite phases by applying driving signals with the same frequency thereto. That is, the ultrasonic generation element 201 vibrates in a buckling tuning-fork vibration mode, and ultrasonic waves are generated from each of the first piezoelectric vibrator 203 and the second piezoelectric vibrator 204.
The ultrasonic generation device 300 further includes a housing composed of a substrate 207 and a cover member 208. The ultrasonic generation element 201 is mounted on the substrate 207 with pillow members 209, such as conductive adhesive, so that a gap is formed between the ultrasonic generation element 201 and the substrate 207. Further, the cover member 208 is bonded to the substrate 207. The cover member 208 has ultrasonic emission ports 208b from which the ultrasonic waves generated by the first piezoelectric vibrator 203 and the second piezoelectric vibrator 204 are emitted outside.
An acoustic path R201 is defined by a gap formed between the first piezoelectric vibrator 203 and the substrate 207 and a gap formed between an outer peripheral surface of the ultrasonic generation element 201 and an inner surface of the housing composed of the substrate 207 and the cover member 208. An acoustic path R202 is defined by a gap formed between the second piezoelectric vibrator 204 and the cover member 208. When the ultrasonic generation element 201 is driven, ultrasonic waves generated by the first piezoelectric vibrator 203 and ultrasonic waves generated by the second piezoelectric vibrator 204 reach the ultrasonic emission ports 208b via the acoustic path R201 and the acoustic path R202, respectively, and are combined into ultrasonic waves having a high output sound pressure. The ultrasonic waves are emitted outside from the ultrasonic emission ports 208b.     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-297219
However, in the above-described ultrasonic generation device 300 whose patent application has been filed by the present applicant (not laid open), a zone where the output sound pressure becomes minimal exists at a frequency comparatively close to a frequency where the output sound pressure becomes maximal in the frequency-sound pressure characteristics. Hence, there is a problem in that the output sound pressure rapidly may fall according to the assembly accuracy, tolerance of components, or temperature change.
FIG. 12 shows the frequency-sound pressure characteristics of the ultrasonic generation device 300. As shown in FIG. 12, a peak of the sound pressure where the output sound pressure becomes maximal (hereinafter referred to as “low-frequency side peak Lp”) exists near 40 kHz, a peak of the sound pressure where the output sound pressure becomes maximal (hereinafter referred to as “high-frequency side peak Hp”) exists near 46 kHz, and a zone where the output sound pressure becomes minimal (hereinafter referred to as “anacoustic zone Ns”) exists between the low-frequency side peak Lp and the high-frequency side peak Hp. The frequency-sound pressure characteristics are obtained by calculating the sound pressure at a position 20 cm apart from the ultrasonic generation device by FEM (finite element method) (this also applies to other graphs of “frequency-sound pressure characteristics” in the present application documents). However, since it is assumed that the amplitude of the vibrator is fixed over the entire frequency range to clarify the influence degree of resonance, the influence of resonance of the vibrator is not reflected herein.
The low-frequency side peak Lp is formed by resonance of air using the vicinity of a vibration surface of the first piezoelectric vibrator 203 as an antinode and each of the ultrasonic emission ports 208b as a node. At this time, ultrasonic waves that are generated in the first piezoelectric vibrator 203 and propagate through the acoustic path R201 and ultrasonic waves that are generated in the second piezoelectric vibrator 204 and propagate through the acoustic path R202 are in phase with each other.
The anacoustic zone Ns is formed when the ultrasonic waves that are generated in the first piezoelectric vibrator 203 and propagate through the acoustic path R201 and the ultrasonic waves that are generated in the second piezoelectric vibrator 204 and propagate through the acoustic path R202 are opposite in phase.
The high-frequency side peak Hp is formed by resonance of air using the vicinity of a vibration surface of the second piezoelectric vibrator 204 as an antinode and the vicinity of each of the pillow members 209 as a node. Although the resonance itself occurs within the ultrasonic generation device 300, since the vicinities of the ultrasonic emission ports 208b are open ends, ultrasonic waves having a comparatively high output sound pressure are emitted from the ultrasonic emission ports 208b. At this time, the ultrasonic waves that are generated in the first piezoelectric vibrator 203 and propagate through the acoustic path R201 and the ultrasonic waves that are generated in the second piezoelectric vibrator 204 and propagate through the acoustic path R202 are opposite in phase.
The ultrasonic generation device 300 most efficiently emits ultrasonic waves when the ultrasonic generation element 201 is driven at the frequency of the low-frequency side peak Lp where the output sound pressure becomes maximal. However, since the frequency of the low-frequency side peak Lp and the frequency of the anacoustic zone Ns are comparatively close to each other, as described above, there is a problem in that the output sound pressure rapidly may fall according to the assembly accuracy, tolerance of components, or temperature change.